The inflammatory response is an attempt by the body to restore and maintain homeostasis after infection or injury, and is an integral part of body defense. Most of the body defense elements are located in the blood and inflammation is the means by which these elements leave the blood and enter the tissue around the injured or infected site. The primary objective of inflammation is to localize and eradicate the source of injury or infection and repair tissue surrounding the site of injury or infection.
As a consequence of the initial innate immune response to infection, phagocytes such as mast cells in the damaged tissue release a variety of cytokines and inflammatory mediators, such as histamines, leukotrienes, bradykinins, and prostaglandins. These inflammatory mediators reversibly open the junctional zones between the thin delicate cells of the inner surface of the blood vessels, known as the endothelium, that surround the damaged tissue. The inflammatory mediators also cause increased blood vessel permeability and decreased blood flow velocity. Another result of these changes in the blood vessels is that leukocytes, which normally travel in the center of the blood vessel, move out to the periphery of the inner surface of the blood vessel to interact with the endothelium. The cytokines and inflammatory mediators released by the phagocytes also induce the expression of adhesion molecules on the surface of the endothelium, resulting in an “activated” endothelium.
The first contact of leukocytes with the activated endothelium is known as “capture” and is thought to involve the adhesion molecules P-selectin and L-selectin, which are upregulated on endothelium after exposure to inflammatory mediators. P-selectin and L-selectin belong to a family of adhesion molecules called selecting. Selectins are a group of monomeric, integral membrane glycoproteins expressed on the surface of activated endothelium and leukocytes. Selectins contain an N-terminal extracellular domain with structural homology to calcium-dependent lectins, followed by a domain homologous to epidermal growth factor, and nine consensus repeats (CR) similar to sequences found in complement regulatory proteins. There are three primary selectins thought to be involved in the inflammatory response: P-selectin; E-selectin; and L-selectin. P-selectin, also known as CD62P, GMP-140, and PADGEM, the largest selectin, is expressed on activated endothelium; E-selectin, also known as ELAM-1, is expressed on endothelium with chemically or cytokine-induced inflammation; L-selectin, also known as LECAM-1, LAM-1, Mel-14 antigen, gp90mel, and Leu8/TQ-1 antigen, is the smallest selectin and is found on most leukocytes. All three selectins are thought to bind to selectin binding ligands, at least in part through a carbohydrate component.
During capture, P-selectin is thought to bind to its main leukocyte ligand P-selectin glycoprotein ligand-1 (PSGL-1). Other ligands of P-selectin include CD24 and yet uncharacterized ligands. The structure of functional PSGL-1 includes a sialyl-Lewisx component. In addition, during capture L-selectin is thought to bind to its ligand on endothelial cells. L-selectin interacts with three known counter receptors or ligands, MAdCAM-1, GlyCAM-1, and CD34, although the precise ligand or counter receptor involved in capture is unknown.
Once leukocytes are captured, they may transiently adhere to the endothelium and begin to “roll” along the endothelium. The term “rolling” refers to the literal rolling of leukocytes along the activated endothelium in the presence of fluid drag forces arising from the relative movement between the endothelium and the leukocytes. Rolling is thought to involve P-selectin, L-selectin, and E-selectin. Bonds between P-selectin and PSGL-1 are thought to primarily mediate the “rolling” of leukocytes across the endothelium.
Proinflammatory cytokines such as interleukin-1 (IL-1), and tumor necrosis factor-α (TNF-α) produced by cells at the injured or infected site stimulate the endothelium to produce chemokines such as interleukin-8 (IL-8) and integrin binding ligands such as intercellular adhesion molecules (ICAMs) and vascular cell adhesion molecules (VCAMs) on the the surface of the endothelial cells opposite the basal lamina. The chemokines are held on the surface of the endothelial cells opposite the basal lamina where the chemokines interact with chemokine receptors on the surface of the rolling leukocytes. This interaction, in turn, triggers the activation of molecules called integrins on the surface of the leukocytes. Integrins are a family of heterodimeric transmembrane glycoproteins that attach cells to extracellular matrix proteins of the basement membrane or to ligands on other cells. Integrins are composed of large α and small β subunits. Mammalian integrins form several subfamilies sharing common β subunits that associate with different α subunits. ∃2 integrins (the “CD-18 family”) include four different heterodimers: CD11a/CD18 (Lymphocyte Function-Associated Antigen-1 (LFA-1)); CD11b/CD18 (Mac-1); CD11c/CD18 (p150,95), and CD11d/CD18. The most important member of the ∃1 integrin subfamily on leukocytes is Very Late Antigen 4 (VLA-4, CD49d/CD29, ∀4∃1). Activation of these integrins by chemokines enables the slowly rolling leukocytes to “arrest” and strongly bind to the endothelium's ICAMs, VCAMs, and other integrin binding ligands of the endothelial cells, such as collagen, fibronectin, and fibrinogen. Once bound to the endothelial cells, the leukocytes then flatten and squeeze between the endothelial cells to leave the blood vessels and enter the damaged tissue through a process termed “transmigration.” Transmigration is thought to be mediated by platelet endothelial cell adhesion molecule-1 (PECAM-1), junctional adhesion molecule (JAM), and possibly CD99, a transmembrane protein.
Despite their importance in fighting infection and injury, leukocytes themselves can promote tissue damage. During an abnormal inflammatory response, leukocytes can cause significant tissue damage by releasing toxic substances at the vascular wall or in uninjured tissue. Alternatively, leukocytes may stick to the capillary wall or clump in venules to such a degree that the endothelium becomes lined with these cells. Such a phenomenon, referred to as “pavementing,” may be related to the development of arteriosclerosis and associated diseases. Such abnormal inflammatory responses have been implicated in the pathogenesis of a variety of other clinical disorders including adult respiratory distress syndrome (ARDS); ischemia-reperfusion injury following myocardial infarction, shock, stroke, or organ transplantation; acute and chronic allograft rejection; vasculitis; sepsis; rheumatoid arthritis; and inflammatory skin diseases.
Several methods and devices exist in the art to study the processes of leukocyte migration implicated in these various inflammatory diseases. For example, one method involves plating a monolayer of isolated endothelial cells on the surface of microtiter plates, activating the cells with a chemoattractant and then placing labeled leukocytes in the plate. A test agent, such as an adhesion inhibitor, may be optionally added to the plate. The number of leukocytes that remain adherent to the endothelial cell monolayer is then determined. A significant disadvantage of this method is that the leukocytes are not exposed to the endothelial cells in the presence of shear flow and thus this method does not simulate physiological conditions in vivo.
Another method involves contacting a suspension of isolated leukocytes in a suitable medium with a human vascular tissue sample mounted on a microscope slide and then incubating the tissue with a cell suspension on a rotating table. The adhered cells are fixed and counted. Because cells are fixed, such a method precludes the observation of leukocyte migration in real time. In addition, such a method requires human vascular tissue, which can be difficult and costly to obtain.
Another method known in the art to study leukocyte migration, involves a device consisting of two glass tubes called microslides, one microslide capable of being inserted into the other. The smaller microslide is inserted into the larger one to create a flow channel with a flat surface on which selected adhesion molecules are present. A suspension of leukocytes is then perfused through the flow channel over the adhesion molecule immobilized surface using a syringe pump. The rolling and adhesion of the leukocytes is then observed. Because of the size and configuration of this device, it requires considerable handling and manipulation.
Another device to study leukocyte migration during the inflammatory response is described in U.S. Pat. No. 5,460,945 to Springer et al. entitled “Device and Method for Analysis of Blood Components and Identifying Inhibitors and Promoters of the Inflammatory Response.” This device consists of several different components that are bulky in size. As such, it requires extra handling and positioning, creating the risk of contaminating or damaging the endothelial monolayer. This device also requires the use of a large number of cells and consequently a large amount of reagents.
Therefore, there exists a need for an improved device to study the leukocyte migration along the endothelium that simulates the physiological conditions of a blood vessel. There also exists a need for a device that would allow for high throughput screening of test agents that potentially affect the interaction of leukocytes with the endothelium without requiring the number of leukocytes per assay as required by the devices currently known in the art. The present invention meets these needs.